DE19925203B4 - Process for welding plastics, and use of the plastic - Google Patents

Abstract

Method for welding at least two plastics, in which the process energy required for welding is laser light energy, characterized in that auxiliary energy is supplied to the joining area, that particles present in the joining area in the joining area reversibly convert at least one substance A _j into one or more substances B _i the substance or substances B _i in their entirety absorb the laser radiation more than substance A.

Description

The invention relates to a method for welding of plastics according to the preamble of claim 1, and the use of a plastic according to claim 9. The invention can then always be used with advantage when welding plastics have to, the for visible light are largely transparent, and the infrared light do not absorb or absorb only weakly. With these plastics generally aimed for, the color change caused by the welding process to avoid. Such requirements exist in packaging, Fashion, automotive and electronics industries as well as in the fashion industry. For example fashionable glasses, glasses lenses, or even contact lenses with filter properties for the Infrared radiation welded become. In the electrical industry, the invention can be reversible optical recording media such as rewritable CDs can be used. Also visors and optical devices counting to the areas of application. Can also be used for windows for cars and houses the invention can be used.

The welding of plastics by high energy radiation, and especially by laser radiation, is known. At least one of the two joining partners must for the high-energy radiation permeable be so for the welding process required Process energy in the joining area can be coupled. All plastic welding processes have in common that the for the joining necessary process energy only in a small area within the joining zone is introduced. The process energy leads to a melting of both welding partner on their surface. If the two welding partners consist of miscible materials, such as PMMA and ABS, the thermal expansion of the melt leads to a material connection with high, barely visible weld quality. For this reason it is weld together of plastics with laser radiation particularly advantageous, if the welding result high optical requirements are made. This is special then the case when the plastics for radiation from the visible Part of the wavelength spectrum are not opaque or transparent. Examples of this are the a colored, for visible Light-transparent plastics such as rear lights of motor vehicles. Another example is colorless transparent Plastics such as plastic eyeglass lenses or Plexiglas.

The absorption of sweat radiation in the joining area can be done in two ways. One possibility is that the second welding partner inherently strongly absorbs the laser radiation, so that it is in the joining area through heat conduction for melting both joining partners comes. This procedure becomes common selected for lap welding.

Another possibility is that colored pigments or carbon black are mixed into a joining partner, which selectively absorb the laser radiation. Such is in the DE 44 32 081 A1 described. However, this procedure disadvantageously leads to the color of this joining partner being changed. Plastic that appears colorless in visible light, such as acrylic glass (also called plexiglass), is given a color, which is undesirable depending on the application. Transparent and colored plastics, and in particular weakly colored plastics, change their color as a result of these pigments or acquire a visually unappealing mixed color. The same problems arise when the pigments are introduced between the two joining partners, for example as a wire, foil or as an absorbent layer.

The DE-OS 14 79 686 also describes a plastic welding process in which, among other things, an absorption increase in the joining area is realized by means of a plasma etching process in order to provide polymer-to-polymer connections between the joining partners.

In the technical article (Opto & Laser Europe, issue 62, p. 16, May 1999) is reported to address the above problems by embedding effective ones in the infrared (IR) spectral range Solve paint particles in the plastic. The IR color particles lead however here too undesirable color changes of the plastic.

The DE 38 10 722 A1 further discloses the use of photochromic polymers as a storage medium for devices for reversible optical information storage.

The possibilities known from the prior art, the absorption of welding radiation in the joining area make sure they have in common that absorption by chromophores, that is, by atomic groupings, that of a compound Color by selective light absorption. Such chromophores Groups are either already present in the plastic (e.g. C = C), or become one of the joining partners later supplied (for example, carbon atoms in soot with strongly delocalized π electrons). The exclusive Using these chromophores makes it impossible according to the prior art to use non-opaque or opaque plastics without color changes to weld.

The invention has for its object to provide a method for welding plastics which avoids the problems of the prior art and which can also be used when the welding radiation is not absorbed by the welding partners.

In particular, the process should also be applicable when both welding partners are colorlessly transparent or are slightly colored and transparent. In particular, the joining partner from the welding process without color changes emerge.

Another job is a plastic welding process to disposal to provide, in which the use of colored pigments is dispensed with can be.

It is also the task of the new Use a plastic to teach the at least one reversibly contains convertible fabric Aj, what about in the wavelength range from 800 - 2000 nm increased absorbency of the plastic leads.

It should also be provided a plastic, the welding radiation in the NIR wavelength range particularly strongly absorbed.

These tasks are accomplished by the invention the in the claims 1 and 9 given features solved. Advantageous configurations for the execution of the method, and the use of the plastic are specified in the subclaims.

According to the invention, it was recognized that the problems can be solved by supplying auxiliary energy to the joining area, that particles of at least one substance A _j present in the joining area are reversibly converted into one or more substances B _i by the auxiliary energy, the substance or substances B, in their entirety absorb the laser radiation more than the substance A _j .

In principle, it is possible for several substances A _j to be present in the joining area, each substance A _{j being} convertible into at least one substance B _i . Here i and j indicate the respective substance with i = 1..n and j = 1..m, where m stands for the number of substances A _j present. The index n stands for the number of substances B resulting from a substance A _j . For example, if there is exactly one substance A, which is converted into three substances B ₁ , B ₂ , B, these three substances B ₁ , B ₂ , B ₃ absorb the sweat radiation overall more strongly than the substance A. Of course it is sufficient in the sense of the present invention if only one of these substances, for example B ₂ , absorbs the sweat radiation more than the substance A ₁ . For the sake of simplicity, we shall only speak of one substance A or B without restricting the inventive concept.

This way, two opaque plastics welded together be where the welding radiation not or only negligibly small is absorbed. Substance A must be in at least one other Fabric B with for the high energy radiation with a larger absorption coefficient be convertible to perform its absorption increasing function. The The process energy is then coupled in through the increased absorption the welding radiation through substance B. The color of the particles of substance A is for this Case irrelevant.

Also welding two or more plastics is possible at least one of which is not opaque. The process energy is coupled in then by increased Absorption of welding radiation through fabric B. Is the non-opaque plastic for visible Light is only slightly transparent, so a colored fabric A can be used become. If the non-opaque plastic is highly transparent, this way, a color that is only weakly colored or even better is colorless Fabric A selected about color changes the welding partner to avoid. The introduction of visible light largely or completely colorless fabric particles ensures that before welding the or the plastics remain unchanged in color. The reversibility of this Conversion process ensures that this is also for the time being after welding applies.

The introduction of this substance A can done in different ways. One possibility is that the material of one of the two welding partners contains this substance. This Plastic must at least be the reversibly convertible substance in the the welding partner facing surface layer and thus in the joining zone contain. This can be done by making the fabric during manufacture of the plastic as a filler is admixed. Naturally is it also possible that several of these substances are mixed in. Through the mixing process automatically becomes an even distribution of the substance in the plastic. A flawless weld with reproducible quality is then chosen by suitably Parameters of welding radiation ensured.

With regard to the wavelengths used welding radiation from λ = 800-2000 nm it is particularly useful if a plastic with at least one substance A is provided becomes, in which the substance A reversible by supplying energy is convertible into at least one substance B, and in which the substance B radiation in the wavelength range from λ = 800-2000 nm stronger absorbed as the substance A. So that the plastic should be weldable it be a thermoplastic or a thermoelast.

Another way to add the substance consists of the substance as a filler metal between the two joining partners contribute. The filler metal consists of a basic material that contains the substance. The The basic material of the filler metal is advantageously a plastic that is readily miscible with the welding partners. For the It is particularly advantageous if the above mixture is good the base material is the same plastic as one of the two Welding partner.

The above-mentioned objects are thus achieved by using a plastic which is distinguished by the fact that it contains at least one substance A which can be reversibly converted into one or more substances B by supplying energy and where the substance or substances B, as a whole, absorb radiation in the wavelength range of λ = 800-2000 nm more strongly than substance A, the use being in the form of a welding filler material.

The filler metal can be a layer, a foil, or a wire.

A layer containing the fabric can according to the usual known methods according to the prior art on one of the two welding partner be deposited. For example, the layer can be pad printed or applied with a brush.

Foils as welding filler materials are special easy to handle and have the further advantage that at predominantly planar surfaces the joining partner slightly large area rates can be achieved. The use of a wire-shaped Filler material is particularly suitable for uneven joining surfaces, to compensate for these unevenness through the melt formation.

To meet the above requirements of the welding result the base material should be guaranteed for non-opaque plastics of the filler material to be largely colorless transparent. specially at to be welded opaque plastics, a colored base material is also conceivable, if there are no optical changes in the joining area result.

The introduced substance A must be reversible in another substance B with for the high energy radiation with a larger absorption coefficient be convertible. The reversible conversion happens because the joining area suitable auxiliary power supplied becomes. This additional Energy (auxiliary energy) is absorbed by substance A and can be too a chemical reaction or energetic stimulation.

In the event of a chemical conversion comes it to form another substance B for the high-energy or welding radiation higher absorption coefficient. After the welding process the energetically stimulated substance releases its energy, or the other substance B converts to the original substance A.

The reversible conversion can be a reversible chemical conversion, such as in photochromic materials occurs. Are photochromic materials particularly advantageous because some representatives of these compounds a particularly strong increase of the absorption capacity is documented is why, for example, at. Sun glasses or rewritable CD recorders (narrow CD recorder) be used. In this sense, suitable substances or classes of compounds are spirooxazines, spiropyranols, azaindolines, spirooxazines, fulgides, Benzopyrans, naphthopyrans, dehydropyrenes, thioindigo, bipyridines, Aziridines, azobenzenes, xanthenes, azo dyes or silver halides.

Also within the meaning of the invention electrochromic substances are suitable as absorption-increasing substances. Electrochromic Materials can through an external electrical Field change color. The supplied in the sense of the invention Auxiliary energy here is the energy of the electrical field. With the color change it also comes to the desired one Increase of absorptivity in terms of welding radiation.

The selected substance A is either completely colorless, only weakly colored, or colored. The first case is colorless transparent plastics required. With weakly colored or largely colorless plastics can be a weakly colored additive A suffice but is a complete colorless substance A more advantageous. This applies in particular to opaque materials to be welded Plastics. If opaque plastics are to be welded, so can colored fabrics A can also be used.

Substance A should be chosen that substance B is the sweat radiation particularly strongly absorbed, i.e. in plastic welding the usual wavelengths used from approx. 800-2000 nm.

For the conversion from substance A to substance B is supplied with auxiliary energy. This can be thermal energy or energy from a second radiation source. If the conversion by exposure to radiation from a second If radiation source occurs, ultraviolet is particularly suitable (UV) radiation (λ = 100-400 nm), visible light (VIS, λ = 400-800 nm), or near infrared radiation (NIR, λ = 800 to 2000 nm). The below other usable radiation sources are diode lasers, Nd-YAG lasers, or in the NIR wavelength range absorbent lasers. In most applications, this radiation is sufficient to be used in continuous wave mode (CW mode). Just for creation Special energetically excited states should have pulsed radiation necessary his.

The back reaction of substance B to Substance A is either automatic with radiation emission, or is brought about by supplying energy. The supplied Energy can be thermal energy or radiation energy a second radiation source. The substance B gives either thermal Energy or radiation energy in the UV, VIS or NIR range.

If the back reaction of substance B is sufficient Time Lag uses it is possible switch off the energy supply before the actual welding process. If the back reaction takes place quickly, the energy supply can also coincide with the welding process respectively. In the latter case, the back reaction can result in a change the radiation intensity Express (equilibrium reaction).

1 shows two art to be welded fabrics ( 3 ) and ( 4 ), between which there is a wire ( 5 ) with a material A which is transparent in visible light and which is reversible in another material B for the welding radiation ( 1 ) greater absorption coefficient is convertible. Plastic ( 3 ) is first carried out with the help of appropriate focusing optics ( 2 ) of radiation ( 1 ) acted upon by the plastic ( 3 ) is not absorbed. The radiation hits the wire in the joining area ( 5 ). The wire contains the reversible. converting substance A. Substance A converts through absorption of radiation into another substance B, which absorbs the high-energy radiation for the welding process more than the starting material.

An NIR laser (eg diode, Nd: YAG laser) is used for the welding process, the radiation of which is now strongly absorbed in the joining area. Since the wire ( 5 ) is placed so that it is in thermal contact with ( 3 ) and ( 4 ), it is melted and the heat enters the plastics via heat conduction ( 3 ) respectively. ( 4 ). This creates a cohesive connection between ( 3 ) and ( 4 ). Without further action, substance B converts back into substance A.

at 2 Substance A is not in a filler metal ( 5 ), but is in the plastic ( 7 ) contain. After switching off the radiation, the plastic ( 7 ) heated and melted by the absorbed radiation. Due to the thermal contact between the plastics ( 6 ) and ( 7 ) heat becomes plastic ( 6 ) transfer. There is a cohesive connection between the plastics ( 6 ) and ( 7 ).

Claims (13)

Method for welding at least two plastics, in which the process energy required for welding is laser light energy, characterized in that auxiliary energy is supplied to the joining area, that particles present in the joining area in the joining area reversibly convert at least one substance A _j into one or more substances B _i the substance or substances B _i in their entirety absorb the laser radiation more than substance A. A method according to claim 1, characterized in that a photochromic or electrochromic substance A _{j is} converted. Method according to at least one of claims 1 to 2, characterized in that a colorless substance A _{j is} converted. Method according to at least one of claims 1 to 3, characterized in that that as auxiliary energy, radiation energy, thermal energy, or electrical Field energy supplied becomes. A method according to claim 4, characterized in that as Auxiliary energy Radiant energy in the wavelength range between λ = 800 - 2000 nm supplied becomes. Method according to at least one of claims 1 to 5, characterized in that that the auxiliary energy is supplied before the welding process. Method according to at least one of claims 1 to 6, characterized in that that the auxiliary energy is parallel to the welding process supplied becomes. Method according to at least one of claims 1 to 7, characterized in that at least one non-opaque plastic welded becomes. Use of a plastic which contains at least one substance A _j which can be reversibly converted into one or more substances B _i by supplying energy, and that the substance or substances B in their entirety absorb the radiation in the wavelength range of 800-2000 nm more than the material A _j , as a filler metal. Use of a plastic according to claim 9, characterized in that the substance A _{j is} photochromic or electrochromic. Use of a plastic according to at least one of claims 9 to 10, characterized in that the substance A _{j is} colorless. Use of a plastic according to at least one of claims 9 to 11, characterized in that the plastic is transparent. Use of a plastic according to at least one of claims 9 to 12, characterized in that the plastic is colorless transparent is.